The larynx is the principal organ of voice. It is situated in the front of the neck, and forms the prominence sometimes called “Adam’s apple”; it also forms a part of the anterior boundary of the pharynx. At the upper part it has the form of a triangular box, with one angle directly in front. It is composed of nine cartilages moved by muscles, and lined with mucous membrane. Six of its cartilages are in pairs; three are single. The three single cartilages are the thyroid, cricoid, and epiglottis; the three pairs are the arytenoid, cuneiform, and cornicula laryngis. The larynx is sometimes called a music-box; from it proceeds the sound called voice.

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VOCAL CORDS.

Across the larynx are stretched the true vocal cords.

Each cord consists of a band of yellow tissue, covered by mucous membrane.

By means of the action of the muscles of the larynx that connect with the cartilages which enter into its structure, the vocal cords are so adjusted that when the muscles of expiration force the air, which is compressed in the lungs, out between these cords, their edges are set in vibration. This is the beginning of the sound which we call voice, but before it is heard in speech or song it is reinforced by the chambers of resonance.^[1]

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[1]	For the function of the false, or superior vocal cords, see pp. 68-71, Physical Culture.

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PITCH.

The various degrees of pitch in the compass of the voice depend upon the rate of vibration of the vocal cords. This rate of vibration, the pressure of breath being the same, is caused by the different degrees of tension of the vocal cords. If the vocal cords are drawn thin and short, the pitch will be high; as the tension diminishes, the pitch will be lower. The greater the number of vibrations to the second, the higher will be the pitch. A sound consisting of sixteen vibrations to the second produces the lowest pitch that has been recognized by the human ear as sound; while more than 38,000 vibrations per second have not been heard.

The lowest rate of vibration on record of any voice is about forty-four vibrations per second, while the highest rate in any voice on record is a little over nineteen hundred.

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LOUDNESS.

Different degrees of loudness of voice are caused by different degrees of amplitude of the waves of vibration.

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THE LUNGS.

The two lungs are the essential organs of respiration; the right lung has three lobes, the left, two. The base of each lung rests upon the convex surface of the diaphragm.

The root of each lung is formed by the bronchus and blood-vessels, which enter the lung a little above the middle of its inner surface, and connect it to the heart and trachea. With the exception of the root, the surface of each lung is free and moves in the cavity of the thorax. The bronchus is one of two tubes which arise from the bifurcation of the trachea. It conducts the air from the trachea to either lung. The bronchial tubes are sub-divisions, or ramifications, of the bronchus and terminate in the air-cells.